Surface Layer Characterization of Atomized Magnesium for use in Powder Metallurgy Products Paul Burke and Georges J. Kipouros

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1 Surface Layer Characterization of Atomized Magnesium for use in Powder Metallurgy Products Paul Burke and Georges J. Kipouros Materials Engineering Program Process Engineering and Applied Science Dalhousie University 1360 Barrington St., Halifax, NS, B3J 2X4

2 Outline Introduction Background Surface Contaminants Mg Surface Contaminants Mg Sintering Strategies Future Work Acknowledgments

3 Introduction Benefits of Magnesium High stiffness to weight ratio High damping capacity Recyclable Aluminum Issues with Magnesium Lack of developed alloys Difficult forming Corrosion Magnesium

4 Introduction Powder Metallurgy Powder metal feedstock Compacted at high pressure in specially shaped die Sintered at temperature below melting Near-net shape parts Mg P/M largely unexplored

5 Stages of sintering Sintering of Metal»Point contact (A) Powders»Initial stage (B)»Intermediate stage (C)»Final stage (D) A B C D

6 Surface Contaminants on Metal Powders Metal core Surface layer Oxide? Hydroxide? Carbonate? Thickness?

7 Surface Contaminants Characterizing surface layer Auger electron spectroscopy (AES) X-ray photoelectron spectroscopy (XPS) Secondary ion mass spectroscopy (SIMS) Transmission electron microscopy (TEM)

8 Surface Contaminants and Sintering Dealing with the surface layer Dissolve layer into metal Diffuse metal through layer Thermo-chemical reduction

9 Surface Contaminants and Sintering Dissolve layer into metal Depends on oxygen solubility in metal Sintering preceded by incubation period Fe ~10 seconds Al ~ 100+ days (Estimate) Mg ~100+ days (Estimate)

10 Surface Contaminants and Sintering Diffuse metal through layer Depends on diffusion rates and layer thickness D M m 2 sec -1 D Ox m 2 sec -1 Cu 5.65 x x Al 1.84 x x Mg 3.01 x x 10-24

11 Surface Contaminants and Sintering Thermo-chemical reduction Free energy diagram Appropriate temperature, pressure and atmosphere Addition of more reactive metal (Mg in Al)

12 Surface Contaminants and Sintering Break layer Create short-circuit pathway for diffusion through cracks in layer Accomplished mechanically or chemically

13 Mg Surface Contaminants When exposed to air, MgO forms with Mg(OH) 2 on the surface Thickness depends on exposure time 1 min, 2.65 nm 7 days, 5.31 nm 7 years, 5.67 nm XPS of Mg Surface

14 Research Objective Fundamentals of magnesium sintering Composition and thickness of surface layer by AES, XPS, SIMS, FIB/TEM Decomposition reactions during sintering by DSC, DTA, TGA Practical strategies to aid sintering and mechanical properties

15 Mg Atomized Powder Surface Contaminants (TEM)

16 Mg Atomized Powder Surface Contaminants (XPS) 3400 C 1s C1s XPS identified that the surface contained: Counts / s C1s A C1s B C1s C C1s D Magnesium Oxygen Carbon Binding Energy (ev)

17 Mg Sintering Strategies (Increased sintering time) Pure Mg 600ºC 6 hours 40 min

18 Mg Sintering Strategies (Thermo-chemical reduction) Elements with more stable oxides: Yb Be Dy Ca Y

19 Mg Sintering Strategies (Thermo-chemical reduction) 1 wt% Y 600ºC 40 min

20 Mg Sintering Strategies (Thermo-chemical reduction) 1 wt% Ca 600ºC 40 min

21 Mg Sintering Strategies (Post Sintering Forging) Samples of 95% or greater density will withstand hot or cold rolling to further increase density 50% cold reduction, 150% hot reduction Density increases near theoretical Large increase in hardness

22 Mg Sintering Strategies (Sintering Atmosphere) Compacts can be sintered in argon or nitrogen During sintering, gas can become trapped as porosity closes Argon completely inert, pressure inside pore prevents densification Nitrogen may react, reducing pressure inside pore

23 Mg Sintering Strategies (Powder Pre-Treatment) Before processing, Mg powder can be dipped with a solution designed to dissolve the surface layer Acids, bases, organic compounds possible Difficulty arises when solution comes into contact with fresh Mg surface

24 Experiment for FIB/TEM Expose Mg powder (~ 50 μm) to air: One day Two days Five days Seven days Examination by FIB/TEM/EDS. Avoid: Water, oxygen Organic solvent Destruction of film

25 A Micrion-2500 Single Beam FIB System - 5 nm imaging resolution using a focused Ga ion beam. - Beam current ranges from 1 pa to 40 na. - Stress free site specific cross-sectioning and imaging. - Gas assisted etching and precise metal and oxide deposition. -Secondary electron (SE) and secondary iron imaging.

26 Schematic Diagram of a Typical FIB System Focused Ion Beam Microscope Ga + Detector e - e - i + e - e - e - e - e - Electron Flood Gun i + Etching/Deposition Gas i + e - e - e - e - i + e - e - e - e - e - e - e - e - Gas e - e - e - e - Gas Gas Gas Gas Specimen

27 Focused Ion Beam (FIB) Microscopy The FIB microscopes were developed in the early 1980s. It has been widely implemented in the semiconductor industry as semiconductor device modification, device failure analysis. In recent years, FIB found many applications in materials studies. Typical materials science applications include: Stress-free ion beam cross-sectioning and highresolution ion beam imaging, site-specific TEM specimen preparation, micro-machining and microdeposition.

28 FIB TEM Sample Preparation Techniques Traditional H-bar technique

29 Lift-out FIB TEM Sample Preparation Technique

30 A Mg particle is mounted onto the edge of the TEM grid using an external lift-out tool

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34 FIB secondary electron image showing the thin film formed on the Mg particle

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40 Future Work Continue FIB/TEM and XPS to identify layer constituents and thickness Confirm findings with AES, SIMS Determine decomposition reactions by thermal analysis (DSC, DTA, TGA) Identify reduction mechanisms of Ca and Y Add alloying additions for liquid formation, strengthening and corrosion resistance

41 Acknowledgments Natural Sciences and Engineering Research Council (NSERC) of Canada Minerals Engineering Centre (MEC) Dr. Jian Li, Dr. C. Bibby (NRCan) Dr. Craig Bennett, Dr. Zeynel Bayindir

42 Thank you